1. Field of the Invention
The present invention relates generally to a plasma etching method of locally etching a relatively thick portion of an article such as a wafer for enhancing flatness or planarization of the article. The invention is also concerned with a plasma etching system for carrying out the method mentioned above.
2. Description of Related Art
Techniques for etching a surface of an article such as that of a silicon wafer or the like are generally known. For example, there are known methods of machining or polishing the whole surface of the wafer by exposing it to a gas of activated species produced through excitation to a plasma state, methods of forming circuit patterns by etching those regions exposed through a mask with a gas of activated species and so forth.
In recent years, there have also been developed techniques for eliminating geometrical variations such as total thickness variation (TTV for short), local thickness variation (LTV in abbreviation) and the like by removing relatively thick portions formed in a surface of an article of concern such as a silicon wafer and a silicon-on-insulator device (SOI for short) in place of the techniques for etching the whole surface of the article such as a wafer. For more particulars, reference may be made, for example, to the technique disclosed in Japanese Unexamined Patent Application Publication No. 5571/1994 (JP-A-6-5571).
For having better understanding of the concept underlying the invention, description will first be made in some detail of the technical background thereof. FIG. 20 of the accompanying drawings is a schematic diagram for illustrating the principle of a conventional plasma etching system known heretofore.
Referring to FIG. 20, reference numeral 100 denotes a plasma generator. A gas G of activated species (hereinafter also referred to simply as the activated gas) contained in the plasma produced by the plasma generator 100 is jetted or ejected from a nozzle 101 onto a surface of a wafer 110, a typical one of the articles which are to undergo etching process.
The wafer 110 is fixedly disposed on a stage 120. By moving or displacing a stage 120 in a horizontal direction, a portion 111 of the wafer 110 which is relatively thicker than a prescribed dimension (this portion is referred to as the relatively thick portion for the convenience of description) is placed at a position immediately beneath the nozzle 101.
Subsequently, the activated gas G is ejected against the relatively thick portion 111 of a convex shape from the nozzle 101, to thereby etch away locally that portion 111. In this way, the surface of the wafer 110 is flattened or planarized.
In this conjunction, it is however noted that the thickness of the relatively thick portion 111 of the wafer 110 is not always uniform but usually diversified.
Under the circumstances, there has been developed a plasma etching system which is so arranged as to control a dwell time of the nozzle 101 (i.e., a time for which the nozzle 101 is caused to stay above the relatively thick portion) in dependence on the thickness of the relatively thick portion 111. By way of example, reference may be made to Japanese Unexamined Patent Application Publication No. 160074/1993 (JP-A-5-160074). According to this known etching technique, the position and the thicknesses of the relatively thick portions 111 are measured over the whole surface of the wafer 110 to thereby prepare corresponding data. The data are then translated into position/dwell-time map data indicating the position(s) of the relatively thick portion(s) 111 and the dwell time of the nozzle 101 which allows a desired degree of flatness to be realized by removing the relatively thick portion 111 through the etching process.
On the basis of the position/dwell-time map data, the stage 120 is so controlled as to move the nozzle 101 to a position immediately above a predetermined relatively thick portion 111 to cause the nozzle 101 to stay at that position for a predetermined dwell time. In this manner, the relatively thick portion 111 can be removed by a desired etch quantity. In this conjunction, with the term "etch quantity", it is contemplated to mean a quantity indicating an amount of wafer material removed by etching.
More specifically, for the relatively thick portion 111 having a greater thickness, the dwell time of the nozzle 101 is extended to thereby increase the etch quantity, while for the relatively thick portion 111 of a smaller thickness, the dwell time of the nozzle 101 is shortened to decrease correspondingly the etch quantity, in an effort to realize planarization over the whole surface of the wafer 110.
The conventional plasma etching technique mentioned above however suffers a problem that due to variations of ambient or environmental parameters such as plasma parameters, realization of the planarization as desired encounters a great difficulty and it is rendered impossible in some case.
In this conjunction, it is noted that the conventional plasma etching technique is based on the presumption that the rate of etching reaction with the activated gas G ejected or jetted from the nozzle 101 is constant and hence the amount of material removed per unit time, i.e., etching rate, is always constant.
In practical applications, however, the ambient or environmental factors such as the plasma parameters are likely to vary, which, of course, brings about variations in the density of the activated gas G as well as in the reaction rate on the surface of the wafer 110. To say in another way, such situations may arise that the etch quantity becomes large or small even for a same dwell time, involving nonuniformity in the surface flatness of the finished wafer.
Thus, in manufacturing wafers on a mass-production basis, there will be apparent dispersions or variations in the flatness quality of the finished wafers 110 such as TTV (Total Thickness Variation), LTV (Local Thickness Variation) and the like.